1. Field of the Invention
The present invention relates to cellulose fiber reinforced board material that is resistant to boiling water and to weather, to a binder mixture for use in said board material and to a process for production of said board material.
2. Description of the Prior Art
The production of board material resistant to boiling water and weather (e.g., chip boards of the quality V 100 according to DIN 68763) made of cellulose fiber-containing natural products such as wood fibers, wood chips, wood veneer, bagasse, rice hulls, flax shives, saw dust and other waste products of wood processing and covered with a synthetic resin binder while using pressure and heat constitutes the general status of the prior art. In the past, an alkali-containing phenol formaldehyde condensate in aqueous solution, or a di- or polyisocyanate were proposed as binders for this purpose. Urea and melamine resins are largely eliminated for this purpose of application because of their lack of resistance to hydrolysis. However, even the binders previously known and used for weather resistant cellulose fiber reinforced boards, used generally and preferably in the open air for construction and building purposes, have a number of disadvantages from the viewpoint of their practical applications which shall be described briefly hereinbelow.
In the case of the strongly alkaline phenol resins still used almost exclusively for this purpose, one result is a relatively high water absorption and a consequent swelling in thickness of boards produced therewith. Moreover, and because of the high alkali content necessary for reasons of hardening techniques, there have resulted disturbing efflorescence as well as the emergence of brownish colored liquid components which above all also make the subsequent production of decorations on the surfaces of the board very difficult. Both in the case of covering with a layer of decorative plastic film as well as in the case of lacquering and above all in a moist atmosphere, brownish discolorations, deepening of colors or general changes of the color tone of these decorative surface layers occur frequently.
With regard to the status of the prior art, the present inventors refer also to the statements by K. Ernst, Holz-Zentralblatt, No. 122, Oct. 10, 1976, page 1585:
"Despite their resistance against moisture, boards covered with phenol resin binders show deficiencies in several modes of application. They are based essentially on the hygroscopic salt content of the phenol resin. In the case of these salts, we are dealing with sodium hydroxide (NaOH) and potassium carbonate (K.sub.2 CO.sub.3). While sodium hydroxide is essential for the production of water soluble phenol resols, the potassium carbonate is used as a hardening accelerator in order to increase the profitability of the production of chip boards. As a result of the presence of salts in the phenol resin-V-100-boards, said chip boards, in the case of the action of humid air, obtain a moisture content which is clearly above the values of solid wood. In the case of a longer lasting action of a relative humidity above 90%, even a secretion of water in the chip boards may occur." PA1 (a) a weakly alkaline phenol-formaldehyde condensate resin with a phenolic compound to formaldehyde mol ratio of 1:1.5 to 1:3.0, preferably of 1:1.8 to 1:2.7, and with an alkali content of .ltoreq.15% by weight NaOH or KOH, related to the solids content of the resin; PA1 (b) a di- and/or poly-functional isocyanate in a quantity of from 5% to 25% by weight, related to (a); and PA1 (c) a tertiary alkyl and/or aralkyl amine and/or alkylene polyamine in a quantity of from 0.1% to 5% by weight, related to (a).
The lowering of the alkali content for the purpose of improvement of the deficiencies explained above, which are the result of the presence of high alkali concentrations, has up to the present time encountered insurmountable technical difficulties since the alkali also functions as the accelerator for the heat cure hardening of the binder. In the case of lowering the alkali content to values of .ltoreq.15% by weight solid alkali in the dry binder, the hardening speeds of the phenol resin binders have fallen so greatly that a profitable processing within predetermined pressing cycles has proven to be impossible in practice, especially when interior layers of the board are to be produced with this binder. A satisfactory solution of these technical problems which occur in use of phenol resins as a binder has therefore not yet been achieved.
When using di- and polyisocyanates as binders, it is true that the difficulties discussed above in connection with phenol resin binders do not occur. Weather resistant cellulose fiber reinforced boards are produced which have a low moisture absorption, a favorable swelling behavior and good mechanical characteristics which remain intact even under continued influences of weather conditions. However, the extremely strong adhesion of the isocyanate binders to the surfaces of the compressed boards has proven to be disadvantageous and an impediment for a broad application as a binder, which hitherto has prevented the use of these binders in the cover layers. The use of the isocyanates has therefore occurred, in the few cases which have become known, exclusively as a binder for inside layers, while the cover layers because of the essentially better separating effect, were covered with a phenol resin glue or binder. (See H. J. Sachs, "Isocyanates as Binders for Chip Boards", Holz-Zentralblatt, No. 20, Feb. 16, 1977, pages 295 and 296). The use of special separating agents on the pressure plates suggested for prevention of the adhesion of isocyanates in the cover layer can be accomplished only with great difficulty or at high additional cost as an additional operating step in the mostly fully automated installations for the production of chip boards, for reasons of process engineering.
A further disadvantage of the isocyanate binders is caused by the essentially higher price which very clearly surpasses that of phenol resins. The higher price is not compensated by the somewhat lower requirement of binder, the somewhat higher speed of hardening and thus shorter manufacturing time of the boards, or by the fact that the fibrous cellulose material used need be less completely dried prior to processing than when using the phenol resins as a binder.
It is therefore a primary object of the present invention to provide a boiling water and weather resistant cellulose fiber reinforced board and process for the production thereof which avoid and overcome the disadvantages of the prior art boards and processes discussed above.
It is a further object of the present invention to provide a binder mixture for cellulose fiber reinforced board which overcomes the drawbacks of prior art binders.
Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art therefrom.